This invention relates to and defines an interrelated and integrated rectification and fractionation system, which together comprise a novel system for achieving the desired fractionation with a minimum energy consumption level. The improved system comprises a rectifier tower having a reflux drum mounted at its top together with a multi-refrigerant core exchanger for tower feed and overhead vapor chilling.
There are various and many processes and systems known for rectifier systems and more especially known systems for supplying and controlling the heat in the different areas and steps of the rectifier systems.
Among the examples of such known technology are U.S. Pat. No. 1,932,903 to McKee; U.S. Pat. No. 2,214,790 to Greenewalt; U.S. Pat. No. 2,582,068 to Roberts; U.S. Pat. No. 3,186,182 to Grossman et al; U.S. Pat. No. 3,444,696 to Geddes et al; U.S. Pat. No. 3,555,836, to Schramm; U.S. Pat. No. 4,002,042 to Pryor et al; U.S. Pat. No. 4,270,940 to Rowles et al; U.S. Pat. No. 4,608,068 to Bauer; U.S. Pat. No. 4,657,571 to Gazzi; and U.S. Pat. No. 5,505,049 to Coyle et al.
McKee, U.S. Pat. No. 1,932,903, describes a process which comprises liquefying a gas by dissolving it in a strong solution of an organic acid in which the gas is more soluble than in water. The resulting salt solution is then heated to remove the dissolved gas and the gas so recovered is dried by contacting it with liquefied gas in a dephlegmator and the resulting dried gas is then cooled.
Greenewalt, U.S. Pat. No. 2,214,790 is directed to a separation process adapted for separation of a gaseous mixture in at least a two-stage rectification, with each stage of the rectification being conducted at a different super-atmospheric pressure, using a liquid refrigerant which can be converted to the gaseous state under the conditions employed in the separation. More specifically, the process is especially for the separation of ethylene from gaseous hydrocarbon mixtures. Ammonia is the refrigerant of choice for the separation of the components in the gaseous hydrocarbon mixture.
Roberts, U.S. Pat. No. 2,582,068 describes a method and apparatus for the separation of a gaseous mixture, which is initially at a high pressure and recovering the more volatile constituent. The separation process is particularly adapted for binary gaseous mixtures and for separating and recovering the more volatile fraction in a pure or greatly purified condition.
Grossman et al., U.S. Pat. No. 3,186,182 describes a low temperature, low-pressure system and process for separation of gaseous mixtures and more particularly, for the demethanization of a gas and more particularly a cracked gaseous mixture.
Geddes et al., U.S. Pat. No. 3,444,696 is directed to an improved process for demethanization of a gaseous mixture in order to recover ethylene from a gaseous feed mixture containing substituents both more and less volatile than ethylene. This process comprises subjecting a cooled feed mixture to a fractionator having a rectifying section at the top and a reboiled stripping section below, the feed mixture containing the ethylene. The invention process is described as achieving economic improvement as respects energy consumption required by employing carefully controlled and different temperature levels in the fractionator section and in the rectifying section as well as locations of the introduction of the feedstock and heat removal by heat exchange between the feedstock and the reflux stream in the sections of the fractionator.
Schramm, U.S. Pat. No. 3,555,836 describes a process for separation of an acetylene-rich gaseous mixture and simultaneous production of acetylene, which includes a pre-cooling process carried out in a series of stages at succeedingly lower temperatures. Individual condensates each of which contains a fraction of acetylene are collected from each stage. The condensates so collected are then recombined in a rectification column, to recover the acetylene and a C2 overhead fraction. This overhead is then scrubbed to remove and recover the acetylene.
Pryor et al., U.S. Pat. No. 4,002,042 describes a process for the separation and recovery of a large (major) portion of the feed gas comprising hydrogen, methane, ethylene and ethane. The feed gas is then passed to a dephlegmator for separation into a vapor stream and a condensate stream. The condensate stream is passed to a demethanizer column where it is fractionated into an overhead methane-hydrogen stream and a bottoms product ethylene-ethane stream.
Rowles et al., U.S. Pat. No. 4,270,940 describes an improved system for recovery of ethane and ethylene from demethanized overhead. The uncondensed vapor effluent from the main reflux condenser is subjected to further condensation and accompanying rectification in a dephlegmator. The liquid condensate from the dephlegmator is then returned to the demethanizer column.
Bauer et al., U.S. Pat. No. 4,608,068 describes process improvement steps adapted to the recovery of C3+ hydrocarbons from a feed stream such as a refinery waste gas, having hydrogen and C1 to C5 hydrocarbons, which comprises the steps of cooling and at least partially condensing the feed stream, and separating the partially condensed feed stream into a liquid fraction and a gaseous fraction.
Gazzi, U.S. Pat. No. 4,657,571 describes a multi-stage process for the recovery of the heavy constituents from a high-pressure hydrocarbon gaseous mixture. The steps include cooling and partial condensation of the hydrocarbon gaseous mixture, separation of the liquid thus obtained from the gaseous mixture and feeding it to a fractionation column. A turbo-expansion is employed for the non-condensed gaseous mixture. The liquid condensed on said turbo-expansion of the gaseous mixture is separated and fed to a fractionation column. The heavy constituents are recovered from the bottom of the column. The gases from the initial fractionation step and from the second step of fractionation after turbo-expansion are then separately or together recompressed to the consignment pressure of the treated gases.
Coyle et al., U.S. Pat. No. 5,505,049 describes a process for removing nitrogen from liquefied natural gas using an enhanced surface, reflux heat exchanger. A relatively warm, high pressure liquefied natural gas is directed counter-currently in heat exchange location with a cool low pressure liquefied natural gas stream to chill the high pressure stream and at least partially vaporize the low pressure stream in a reflux heat exchanger. The vapor so produced strips the low-pressure stream of nitrogen. The cool low-pressure stream is produced by expansion of the chilled high-pressure stream. The vapor produced by this expansion is then combined with the vapor which is produced in the heat exchanger and is removed and recovered from overhead of the heat exchanger. A product of liquefied natural gas, which has low nitrogen content is recovered from the bottom of the heat exchanger.
Although many of the above-mentioned prior art processes have met with some success in industry, the energy costs in operating these systems is relatively high. Accordingly, there exists a need in the industry to develop a process which has essentially the same fractionation capability but which reduces energy costs.
According to the present invention there is provided an improved separation process and apparatus, which effect excellent fractionation at reduced energy requirements.
The apparatus of the invention is shown in detail in the accompanying Figure and described in detail herein below. The apparatus comprises a rectifier column which is equipped with fractionation trays, a reflux drum mounted on top of the rectifier tower and an elevated multi-refrigerant core heat exchanger which is used for the tower feed and for the overhead vapor chilling, and a main feed line, which branches into two feed lines, the first of which is directed to a lower portion of the rectifier tower and the second of which is directed through the multi-refrigerant core heart exchanger and then to the rectifier tower at a height above the first feed line.
With respect to the process operation of the system, improved fractionation and separation results with reduced energy requirement, can be achieved with the apparatus of the invention. The hydrocarbon feed stream to the system is initially split into two streams. The first portion of the feed stream is fed into the bottom section of the rectifier tower as stripping vapor. The second portion of the feed stream is chilled in the core heat exchanger and is then passed to the rectifier tower as the tower feed. The tower overhead vapor is also chilled in the core heat exchanger before it is introduced into the reflux drum. The resulting flashed liquid from the reflux drum is returned to the rectifier tower by gravity flow as reflux liquid.